As a laser welding method for welding plastic components, a method is known in which a laser-beam-transmitting plastic component that transmits a laser beam is superposed on a laser-beam-absorbing plastic component that absorbs a laser beam, a laser beam is irradiated from above the laser-beam-transmitting plastic component toward the laser-beam-absorbing plastic component to cause the laser-beam-absorbing plastic component to absorb laser beam energy and generate heat, and the vicinity of abutting faces at which the laser-beam-transmitting plastic component and the laser-beam-absorbing plastic component come in contact are melted by the heat generated in the laser-beam-absorbing plastic component, and then allowed to harden and weld.
FIG. 72 illustrates the principles of a conventional laser welding method for welding plastic components. In FIG. 72, a laser-beam-absorbing plastic plate 302 and a laser-beam-transmitting plastic plate 303 are placed on a supporting base 301 made of glass, and a pressure member 304 made of glass is placed on the laser-beam-transmitting plastic plate 303. The pressure member 304 is pressed downward by an unshown pressing source, for example, an air cylinder, to thereby cause the abutting faces of the laser-beam-absorbing plastic plate 302 and the laser-beam-transmitting plastic plate 303 to closely contact.
Subsequently, when a laser beam 305 is irradiated onto the entire area of the aforementioned abutting faces from above the pressure member 304, the laser beam 305 is transmitted through the pressure member 304 and the laser-beam-transmitting plastic plate 303 and is absorbed by the laser-beam-absorbing plastic plate 302. The laser-beam-absorbing plastic plate 302 absorbs the laser beam 305 and generates heat. In the laser-beam-absorbing plastic plate 302, the area in the vicinity of the abutting face with the laser-beam-transmitting plastic plate 303 which the laser beam 305 impinges upon becomes a high temperature and melts. Subsequently, the abutting face of the laser-beam-transmitting plastic plate 302 also melts. When irradiation of the laser beam 305 stops, the melted portions cool and harden, and thus the laser-beam-transmitting plastic plate 303 and the laser-beam-absorbing plastic plate 302 are welded (see Patent Document 1).
FIG. 73 is a view that illustrates the principles of a laser welding method in the case of performing laser welding of a plurality of laser-beam-transmitting plastic plates 303a and 303b through which a laser beam passes. In FIG. 73, the plurality of laser-beam-transmitting plastic plates 303a and 303b are mutually superposed and placed on the supporting base 301 made of glass. Since the plurality of laser-beam-transmitting plastic plates 303a and 303b transmit the laser beam, the laser-beam-transmitting plastic plates 303a and 303b do not generate heat. Therefore, a laser beam absorbing material 306 that absorbs the laser beam 305 is coated between the respective abutting faces of the plurality of laser-beam-transmitting plastic plates 303a and 303b. A pigment such as carbon black is mixed into the laser beam absorbing material 306.
When the laser beam 305 is irradiated from above the pressure member 304 onto the entire area of the abutting faces of the laser-beam-transmitting plastic plates 303a and 303b on which the laser beam absorbing material 306 is coated, the laser beam absorbing material 306 absorbs the laser beam 305 and generates heat and melts. The laser beam absorbing material 306 acts as an adhesive that bonds the laser-beam-transmitting plastic plates 303a and 303b to thereby weld together the two plastic plates 303a and 303b (see Patent Documents 1 and 2).
According to the above described laser welding methods there are the advantages that (1) abutting faces of a plurality of plastic components are firmly welded, (2) a beautiful finish is achieved, and (3) vibrations are not applied to the components as in the case of ultrasonic welding. For these reasons, the above described laser welding methods are used for miscellaneous kinds of plastic welding (see Patent Document 3).
However, the above described laser welding methods are based on the premise that one of the laser-beam-absorbing plastic plate 302 and the laser beam absorbing material 306 absorbs a laser beam. There has been the problem that because a pigment such as carbon black is included in the laser-beam-absorbing plastic plate 302 or the laser beam absorbing material 306, the plastic plate 302 or absorption material 306 is colored and opaque or, even if the plastic plate 302 or absorption material 306 is semi-transparent, the degree of transparency is low.